At present, it is publicly known that, in order to accommodate remarkably increasing Internet traffic, the introduction of optical fibers into a network which uses wavelength division multiplexing (WDM) communication as a core technique advances at a quick pace. While the WDM at the present time principally assumes the form of a point-to-point network, it is anticipated that the WDM is progressed to a ring type network or a mesh-type network in the near future.
Further, it is anticipated that, in optical transmission apparatus (nodes) which compose the network, it becomes possible to use processes such as adding/dropping (Add/Drop) of an arbitrary wavelength, all-optical cross connect (Optical Cross Connect: OXC) which does not involve conversion into the electricity and dynamic setting and cancellation of a pass based on wavelength information are performed.
A wavelength selection switch is provided in an optical transmission apparatus in a ring type network or a mesh type network and is an optical apparatus having a function of distributing inputted light to an arbitrary output port for each wavelength. The wavelength selection switch is expected as a promising optical apparatus for making it possible to implement such processes of adding/dropping of an arbitrary wavelength and all-optical cross connect without conversion into the electricity as described above and so forth.
As such an optical apparatus which forms a wavelength selection switch as described above, an optical apparatus is available which includes not only a spectral element, a condensing optical system and so forth but also a MEMS (Micro Electro Mechanical System) mirror array for changing over the path. For such an optical apparatus which forms a wavelengths election switch as just described, various techniques for enhancing the degree of freedom in design and improving a characteristic of light to be outputted while coping with standardization by the International Communication Association or the like are developed.
Further, a technique is preferable for enhancing, in an optical system which includes a collimate optical system for collimating a light beam and a condensing optical system coupled to the collimate optical system, the degree of freedom in design and improving a characteristic which an optical system has.    (Patent Document 1) Japanese Patent Laid-Open No. 2006-126566    (Patent Document 2) Japanese Patent Laid-Open No. 2001-84625    (Patent Document 3) Japanese Patent Laid-Open No. Hei 1-108522    (Patent Document 4) Japanese Patent Laid-Open No. Hei 7-284976